A fuel cell stack which constitutes a fuel cell system oxidizes a fuel through an electrochemical process to thereby directly convert energy released due to such oxidization reaction into electric energy. Such fuel cell stack has a membrane-electrode assembly in which a polymer electrolyte membrane, which selectively transports hydrogen ions, is sandwiched by a pair of electrodes made of porous materials. Each of the pair of electrodes includes: a catalyst layer that contains, as a main ingredient, carbon powder supporting a platinum-based metal catalyst, and that contacts with the polymer electrolyte membrane; and a gas diffusion layer formed on a surface of the catalyst layer, the gas diffusion layer having both air permeability and electronic conductivity.
In a fuel cell vehicle in which such fuel cell system is installed as an electric power source, operation in a high-output zone with a high power generation efficiency is controlled such that the fuel cell stack is operated to generate electric power so as to supply electric power to a traction motor from both the fuel cell stack and a secondary battery or from the fuel cell stack alone, while operation in a low-output zone with a low power generation efficiency is controlled such that power generation by the fuel cell stack is temporarily stopped and electric power is supplied to the traction motor only from the secondary battery. Temporarily stopping the operation of the fuel cell stack in a low-load zone with a low power generation efficiency of the fuel cell system, as described above, is called an “intermittent operation”. By performing such intermittent operation in a low-load zone with a reduced power generation efficiency of the fuel cell system, the fuel cell stack can be operated within a range with a high energy conversion efficiency and the efficiency of the fuel cell system as a whole can be enhanced.
Among fuel cell systems that perform such intermittent operation, a known system performs an intermittent operation if a load which is requested to be covered by the fuel cell stack is equal to or lower than a predetermined value. When the fuel cell stack in this fuel cell system is shifted to a power generation stop state as a result of performing an intermittent operation, and if the cell voltage of the fuel cell stack decreases below a predetermined value, the fuel cell system drives an air compressor to supply oxygen gas to the fuel cell stack, in order to recover the cell voltage by solving the shortage of oxygen in the cathode of the fuel cell stack, so as to thereby improve the delay of a response to a power generation request.
In the above-described intermittent operation, the supply of reactant gases to the fuel cell stack is stopped and a command voltage for a DC/DC converter which is connected in parallel to the output terminal of the fuel cell stack is set to an open circuit voltage (OCV) so that the output terminal voltage of the fuel cell stack is controlled to a high-potential avoidance voltage which is a voltage smaller than the open circuit voltage. By maintaining the output terminal voltage of the fuel cell stack at the high-potential avoidance voltage which is smaller than the open circuit voltage, it is possible to control the current flowing out from the fuel cell stack during the intermittent operation.
In the conventional intermittent operation, in order to ensure output responsiveness while maintaining the output terminal voltage of the fuel cell stack at the high-potential avoidance voltage or lower, air blowing control which drives an air compressor to blow in a large amount of air is performed if the output terminal voltage decreases to a predetermined threshold or lower. However, when such air blowing control is performed, the output terminal voltage will increase and exceed the open circuit voltage (OCV), and it is thus necessary to suppress such increase in voltage by driving the DC/DC converter so as to cause the fuel cell stack to generate electric power. Since this power generation is performed for the purpose of maintaining the output terminal voltage at a value equal to or lower than the high-potential avoidance voltage, it is preferable to avoid performing such power generation in view of the efficiency of the entire fuel cell system. Under these circumstances, Patent Document 1, indicated below, describes a fuel cell system capable of suppressing unnecessary power generation by the fuel cell stack while performing high-potential avoidance control during the intermittent operation.
The fuel cell system described in Patent Document 1 has: fuel gas supply means that supplies a fuel gas to an anode; oxidant gas supply means that supplies an oxidant gas to a cathode; and control means that controls the fuel gas supply means and the oxidant gas supply means and causes a fuel cell stack to perform power generation in response to a required power. The control means monitors an output terminal voltage of the fuel cell stack during an intermittent operation so that the output terminal voltage is equal to or lower than an upper limit voltage for the intermittent operation and is equal to or greater than a lower limit voltage for the intermittent operation, and, based on the result of the monitoring, controls the supply of the oxidant gas in such a manner that a small amount of oxidant gas is supplied continuously.